This invention relates to a method of driving an image sensor provided in an image input port of an image scanner, a facsimile equipment, an OCR equipment and the like.
The contact type image sensor optically acts as an image sensor having an optical system with magnification of unity and in order to read a manuscript of, for example, A4 size, this image sensor is required to be so huge that its size is comparable to the lateral width of A4 size. To this end, a plurality of image sensor chips each having a size of, for example, about 7 cm which corresponds to one of three divisions of A4-size lateral width are juxtaposed on a substrate of ceramic or the like. Actually, this image sensor is equivalent to three independent image sensors which are optically interconnected together and for the sake of picking up sensor outputs over the entire A4 size, the individual sensors must be driven sequentially at timings which do not concur or conflict with each other. In a prior art as disclosed in, for example, JP-A-61-161580, one image sensor is relayed to another by simply driving these sensors at offset timings, raising a problem that a noise is superimposed on an image sensor output at a relaying portion.
The aforementioned prior art will be described more specifically with reference to FIG. 1.
Continuous application of a shift clock 101 to chips 1 to 3 is performed on common timing basis. A start signal is applied to a chip and an image output signal occurs subsequently. A start signal 102 for the chip 1, a start signal 104 for the chip 2 and a start signal 106 for the chip 3 are sequentially applied at the predeterminedly dephased relationship so that the delivery of image output signals 103, 105 and 107 may be continuous on the time axis. The continuous delivery of the image output signals from the chips 1 to 3 can be regarded as a continuous sensor output signal or can be handled as an output signal from one unitary image sensor chip.
However, in the prior art, in order to permit the continuous delivery of image output signals from the plurality of chips to be regarded as a continuous sensor output signal, a start pulse 109 for the chip 2, for example, is generated near the timing for an image output 108 of the preceding chip 1. As a result, a noise is superimposed on an end portion of the image output 108 as shown at dotted line in FIG. 1. Similarly, in the image output signals 105 and 107, noises as shown at dotted line are superimposed on end portions. Since the position of the start pulse 109 corresponds to the end portion of the image output from the preceding chip, noise is superimposed on the image output non-uniformly, resulting in a harmful noise which cannot be removed by means of the succeeding image signal processing circuit (not shown). This noise is particularly harmful to the image scanner adapted to receive data representative of light and shade of images with high fidelity.
It should therefore be understood that in the prior art, the plurality of image sensor chips are sequentially driven by simply relaying one chip to another at offset timings and consequently a noise is superimposed on the output signal from the sensor chip.